Processing scheme for domain expansion rom media

ABSTRACT

The present invention relates to a domain expansion storage medium and processing method for processing a substrate ( 40 ) of such a storage medium with improved efficiency. The surface structure of a substrate of the storage medium is processed by an ion beam projection lithography using a stencil mask to define magnetic domains in a storage layer ( 50 ) and/or track patterns and/or servo patterns. Thereby, replication accuracy and speed can be improved.

The present invention relates to a read-only domain expansion storagemedia and a processing scheme for processing a substrate of such mediain which a magnetic wall is displaced to thereby enlarge a magneticdomain so as to reproduce an information indicated by the magneticdomain.

In magneto-optical storage systems, the minimum width of the recordedmarks is determined by the diffraction limit, i.e. by the NumericalAperture (NA) of the focussing lens and the laser wavelength. Areduction of the width is generally based on shorter wavelength lasersand higher NA focussing optics. The capability of writing extremelysmall domains is essential to increasing areal storage densities inmagneto-optical (MO) media. Domain expansion media typically consist ofa polycarbonate substrate, a reflective heat conducting layer, a firstdielectric layer, a magnetically hard e.g. TbFeCo storage layer which iscoupled either magnetostatically through a second dielectric layer ordirectly via exchange coupling through intermediate magnetic layers to amagnetically soft e.g. GdFeCo read-out layer, a third dielectric layerand/or an acrylic resin cover layer. Data storage is achieved by using athermomagnetic writing technique whereby the thin storage layer having athickness of about 20 nm is heated to the Curie temperature by afocussed laser or other radiation spot, and then allowed to cool down inthe presence of a magnetic field. The heated area is thereby “frozen”with a magnetic orientation parallel to that of the magnetic field.Fortunately, writing is a thermal process which is not limited to thespot size of the laser, but rather to the size of the heated area.Currently, the ability to write small domains far exceeds the ability toread them. Writing is achieved by modulating either the laser power,e.g. in Light Intensity Modulation (LIM), the external field, e.g. inMagnetic Field Modulation (MFM), or both, e.g. in Laser Pumped MFM(LP-MFM). Data retrieval is achieved via domain expansion whereby adomain written in the storage layer is copied to the read-out layerwhere it expands to fill the optical read-out spot.

MAMMOS (Magnetic AMplifying Magneto-Optical System) is a domainexpansion method based on magneto-statically coupled storage and readoutlayers, wherein a magnetic field modulation is used for expansion andcollapse of expanded domains in the readout layer. A written mark fromthe storage layer with high coercivity is copied to the readout layerwith low coercivity, upon laser heating with the help of an externalmagnetic field. Due to the low coercivity of this readout layer, thecopied mark will expand to fill the optical spot and can be detectedwith a saturated signal level which is independent of the mark size.Reversal of the external magnetic field collapses the expanded domain. Aspace in the storage layer, on the other hand, will not be copied and noexpansion occurs. Therefore, no signal will be detected in this case.

Domain Wall Displacement Detection (DWDD) is another DomEx method basedon an exchange-coupled storage and readout layer, proposed by T.Shiratori et al. in Proc. MORIS '97, J. Magn. Soc. Jpn., 1998, Vol. 22,Supplement No. S2, pp. 47-50. In a DWDD medium, marks recorded in thestorage layer are transferred to a displacement layer via anintermediate switching layer as a result of exchange coupling forces.The temperature rises when reproducing laser spots are irradiated ontothe discs recording tracks. When the switching layer exceeds the Curietemperature, the magnetization is lost, causing the exchange couplingforce between each layer to disappear. The exchange coupling force isone of the forces holding the transferred marks in the displacementlayer. When it disappears, the domain wall surrounding the recordedmarks shifts to a high temperature section which has low domain wallenergy, allowing small recorded marks to expand. The domain wall whichhad been transferred into the displacement layer shifts as if beingpulled by a rubber band. This allows reading via laser beam, even ifrecordings have been made at high density.

Domain expansion techniques such as MAMMOS and DWDD thus allow readoutof bits much smaller than the size of the optical spot, but with asignal much larger than in MSR. The various disk stacks always comprisea recording layer and a readout layer, which may be coupledmagneto-statically or by means of exchange coupling. RF MAMMOS requiresa modulating external magnetic field during readout, which increases thepower consumption, but also allows readout at very high densities andwith large signals. Alternative techniques like ZF MAMMOS and DWDDrequire no external magnetic field during readout, but are expected tobe limited to somewhat lower densities, smaller signals and lower datarates.

Present domain expansion technology is restricted to re-writable disks.However, a ROM domain expansion solution by which data cannot be freelywritten to the domain expansion medium or disk does not exist. Infamilies of optical storage media, the ROM (Read Only Memory) format isseen as an addition used for cheap and fast reproduction of pre-recordeddata. These properties of ROM are considered essential for the successof an optical storage product-family. In the case of domain expansionmedia, a ROM solution is not trivial. The reason is that data is definedby magnetization directions in the storage layer, which are not easilyreproduced in pre-recorded media, e.g. by injection moulding.

Documents U.S. Pat. No. 5,993,937 and EP 0848381A2 disclose domainexpansion ROM media with a domain expansion stack on an injectionmoulded substrate with smooth and rough areas to define the recordedinformation. Both solutions utilize etching through an e-beam masteredresist pattern in order to roughen areas on a glass master or substrate.The master may then be used to produce conventional stampers which arein turn used to produce substrates with roughened areas. The magneticstorage layer will exhibit an enhanced domain wall coercivity in areaswhere the substrate has been roughened, so that the magnetization inthese areas will be more difficult to erase and much harder to overwritein such a way as to preserve good read-back performance successfully.

One drawback of using the conventional glass master patterning androughening techniques resides in that stampers are required which areexpensive to produce and which have a limited lifetime. Furthermore, asbit sizes decrease to sub-100 nm dimensions, perfect replication of theroughened ROM data pattern will become technically more demanding.Moreover, patterning and roughening of individual substrates byirradiation of resist followed by etching is time consuming due to theserial writing process which may hinder the commercial viability of thistechnique.

It is therefore an object of the present invention to provide a moreefficient solution for processing substrates of domain expansion ROMmedia.

This object is achieved by a processing method as claimed in claim 1 anda domain expansion storage medium as claimed in claim 13.

Accordingly, a high resolution non-contact technique can be used forprocessing the substrate of domain expansion storage media, to therebyenable improved replication of ROM data patterns. During ion beamprojection the mask pattern may be reduced, so that the mask featuresize can be larger than the required minimum medium feature size.

The processed surface may be the surface of the substrate as such or thesurface of an additional layer of a seed metal or a dielectric materialdeposited on the substrate before performing the ion beam projectionstep, wherein the surface of the additional layer is processed in theprocessing step. In the latter case, better control of the surfaceprocessing may be allowed.

The surface processing may be a sputtering process to generate a patternof roughened or smoothed areas at said exposed portions. Whether theexposed areas are roughened or smoothed depends upon the exposure time,energy and mass of the incident ions, and the material being exposed.

Additionally, the processing step may be adapted to modify opticalproperties at predetermined surface portions so as to define a trackstructure of the storage medium. Thereby, the land/groove trackstructure of conventional optical media can be replaced by asmooth/rough track structure. Specifically, a first mask may be used forforming the data pattern, while a second mask may be used for formingthe track structure. The beam projection and processing steps may beperformed at least two times for the track structure.

Furthermore, the beam projection and processing steps may be adapted topattern embedded servo information into said surface. Consequently, alsoin this respect, corresponding land/groove structures of conventionaloptical disks can be dispensed with The focus of the at least one ionbeam may be controlled so as to modify the roughness of the surface.Then, a first focus can be used for forming the data structure, while asecond focus can be used for forming the servo pattern.

A whole disk is patterned in the ion beam projection and processingsteps. Thereby, individual data, track and/or servo patterns can bewritten simultaneously in a short processing time.

The mask may be formed by an e-beam lithography and a subsequentsemiconductor etching.

Further advantageous modifications are defined in the dependent claims.

In the following, the present invention will be described in greaterdetail on the basis of preferred embodiments with reference to theaccompanying drawings, in which:

FIG. 1 shows a schematic diagram of an ion beam projection lithographyarrangement which can be used for the present invention;

FIG. 2 shows a sectional view of a layer arrangement of a domainexpansion storage medium according to a first preferred embodiment ofthe present invention;

FIG. 3 shows a sectional view of a layer arrangement of a domainexpansion storage medium according to a second preferred embodiment ofthe present invention; and

FIG. 4 shows a schematic flow diagram of a substrate processing methodaccording to the preferred embodiments of the present invention.

The preferred embodiments will now be described on the basis of a domainexpansion ROM disc, wherein a substrate is processed duringmanufacturing by using an ion beam projection lithography (IPL).

FIG. 1 schematically illustrates an ion beam projection lithography(IPL) arrangement or tool. In general, such an IPL tool is used for theformation of an image of a structured mask or stencil mask 20, i.e. amask provided with openings 25 for passing a beam of ions, upon asubstrate 40 of the domain expansion ROM disk and comprises an ionsource 10 for generating the ion beam, the structured stencil mask 20and an immersion lens 14 between the stencil mask 20 and the substrate40. The immersion lens 14 serves to accelerate the ions to the desiredfinal energy for structuring the substrate 40. Furthermore, a prelens 12and a projection lens 16 can be provided also in the path of the ionbeam. On the substrate 40, a demagnified pattern 45 can be obtained at asize depending on the projection parameters. The ion source 10 may be ahelium (He) ion source for generating desired He ions. Further detailsof the IPL tool can be gathered from Kaesmaier et al., SPIE conferenceon Microlithography, Santa Clara, Calif. (2000).

According to the preferred embodiments, ion beam projection lithography(IPL) provides an alternative high resolution surface modificationand/or patterning technique for processing the substrate 40. Thistechnique has been used conventionally to pattern magnetic hard diskmedia, whereby the magnetic properties of the media are altered by ionimplantation. If ions with the correct energy (momentum) are used, thenit is also possible for material to be sputtered away from verylocalized areas of the bare substrate, thus leaving a pattern ofroughened or smoothed areas. Subsequent deposition of a MAMMOS stack onthis modified or patterned substrate will result in a DomEx ROM disk.

FIG. 2 shows a sectional view of a layer structure of the domainexpansion ROM disk according to the first preferred embodiment. Ingeneral, the magneto-optical recording medium or disk for realizingsuper-resolution or domain expansion reading may be composed of anymagnetic layer or film differing in the coercive force depending on therecorded information and possessing a relatively large magneto-opticaleffect. The recording information is expressed by roughening therecording domain portions to form rough areas 42 on the substrate 40, ascompared with the remaining smooth areas 44. The coercive force beginsto increase when the mean dimension of the roughness of the surface inthe in-plane direction becomes about 10 nm or more, and the coerciveforce begins to increase when the mean dimension of the roughness of thesurface in the perpendicular direction becomes about 3 nm or more.Hence, depending on the recording information, when a rareearth-transition metal (RE-TM) alloy magnetic storage layer 50 is formedon the substrate 40 having the rough areas 42 with mean roughness of thesurface in the in-plane direction and perpendicular direction of 10 nmor more and 3 nm or more, respectively, and the smooth portions 44 withmean roughness of the surface in the in-plane direction andperpendicular direction of 10 nm or less and 3 nm or less, respectively,a magneto-optical recording medium possessing portions differing in thecoercive force depending on the recording information is obtained. Thestorage layer 50 is covered by readout and dielectric layers 60 to formthe required DomEx layer stack.

FIG. 3 shows a sectional view of a layer structure of the domainexpansion ROM disk according to the second preferred embodiment. Here, aseed metal or dielectric layer 70 is first deposited on the substrate 40and then roughened or patterned using IPL to form rough areas 72 andsmooth areas 74 at the surface of the seed layer 70. This additionalseed layer 70 may allow greater control of the roughness of thepatterned areas.

In both preferred embodiments, it may also be possible to roughen thesubstrate 40 or the seed layer 70, respectively, by IPL to such anextent that the optical properties of the stack may be modified enoughfor detectable variations in reflectivity to occur. This may allow theland/groove track structure of conventional optical media to be replacedby a smooth/rough track structure, any loss in reflected lightindicating track edges. By the use of suitable masks, it may be possibleto “single” expose areas forming the ROM data pattern, and “double”,“triple” or “quadruple”, etc., expose areas forming the track structureby repeating the IPL roughening or patterning process for acorresponding number of times.

Alternatively, in both preferred embodiments, IPL may be used to patternembedded servo information into the surface of the substrate 40 or theseed layer 70, also allowing the land/groove structure of conventionaloptical disks to be dispensed with. Such a servoing technique is similarto that used in hard disks.

Focussed ion beam equipment may also be used to modify the roughness ofthe substrate 40 or seed layer 70 in order to form ROM data, servopatterns and/or track patterns. However, the use of a single focussedion beam that has to move across the whole substrate surface may take aprohibitively large amount of time and therefore be commerciallyunattractive.

The advantage if IPL is that it is a high resolution non-contacttechnique. Therefore, the perfect replication of ROM data patternsshould be considerably eased. Furthermore, a whole small format disk maybe patterned in one exposure with the individual data, track and/orservo patterns being written simultaneously in a number of seconds. Theaim is to take advantage of the 300 mm wafer throughput of up to 50wafers per hour, or more, at the 50 nm lithography node (i.e. resolutionin terms of half pitches and feature sizes), over stitched 12.5 mm×12.5mm fields. Assuming that the time required for exchanging and exposingthe substrates is under 20 seconds, and that the area to be patternedlies within a 12.5 mm diameter circle, a throughput of 180 discs perhour could be achieved. Larger exposure fields may be accommodated,depending upon the level of pattern distortion at the disk edges thatcan be tolerated. Alternatively, by using multiple exposures of adjacentareas, larger disks may be patterned at the cost of disc throughput.During the IPL process a 150 mm SOI (Silicon On Insulator) stencil mask20 pattern can be reduced e.g. by a factor of four during projectiononto the substrate. Therefore, the minimum stencil mask 20 feature sizemay be larger than the required medium minimum feature (bit) size. Thestencil mask 20 itself can be manufactured using e-beam lithography andsemiconductor etching techniques.

In the following, a method of processing the substrate 40 of the domainexpansion ROM disk is described with reference to the flow diagram ofFIG. 4. According to FIG. 4, in step S100, the substrate 40 is firstformed while use can be made, for example, of glass, polycarbonate,polymethyl methacrylate, resin of a thermoplastic origin, or the like.Then, in case of the structure of the second preferred embodiment, theseed or dielectric layer 70 is deposited on the substrate 40 in stepS101. It is noted that step S101 is omitted in the first preferredembodiment. In step S102, material at the surface of the substrate 40 orthe seed layer 70 is sputtered away by IPL to form a pattern ofroughened areas 42, 72 so as to define domain portions in thesubsequently deposited storage layer 50, and/or track and/or servopatterns. Finally, the remaining layer stack of the DomEx ROM disk isformed or deposited on the processed or roughened surface in step S103.

The magnetic storage layer 50 and the magnetic readout layer may becomposed of any RE-TM compound having relatively high magneto-opticaleffects, such as TbFe, GdTbFe, TbFeCo, DyFe, GdDyFe, DyFeCo, GdDyFeCo,and NdTbFeCo, or a transition metal oxide and nitride compound film, aferrite film, or a 3D transition metal magnetic film, includingmultilayers of such films.

The present invention can be applied to any domain expansion ROM medium,while the ion beam processing can be adapted to obtain any suitablesurface structure of the substrate 40, seed layer 70 or otherintermediate layer, which can be used to obtain optical or magneticproperties sufficient to define the proposed domain portions, trackand/or servo patterns. The preferred embodiment may thus vary within thescope of the attached claims.

1. A method of processing a substrate for a domain expansion storagemedium in which a magnetic wall is displaced in a readout layer tothereby enlarge a magnetic domain of a storage layer so as to reproducean information indicated by said magnetic domain, said method comprisingthe steps of: a) passing at least one beam of ions through a mask with apredetermined pattern so as to project said predetermined patterntowards said substrate; b) processing a surface by said at least onebeam of ions at exposed portions; and c) depositing said storage layerabove said processed surface so as to define magnetic domains of a datastructure in said storage layer at portions corresponding to saidexposed portions.
 2. A method according to claim 1, further comprisingan initial step of depositing an additional layer of a seed metal or adielectric material on said substrate before performing said ion beamprojection step, wherein the surface of said additional layer isprocessed in said processing step.
 3. A method according to claim 1,wherein the surface of said substrate is processed in said processingstep.
 4. A method according to claim 1, wherein said surface processingis a sputtering process to generate a pattern of roughened or smoothedareas at said exposed portions.
 5. A method according to claim 1,wherein said processing step is adapted to modify optical properties atpredetermined surface portions so as to define a track structure of saidstorage medium.
 6. A method according to claim 5, wherein a first maskis used for forming said data pattern and a second mask is used forforming said track structure, or vice versa.
 7. A method according toclaim 5, wherein said beam projection and processing steps are performedat least two times for said track structure.
 8. A method according toclaim 5, wherein said beam projection and processing steps are adaptedto pattern embedded servo information into said surface.
 9. A methodaccording to claim 1, further comprising the step of controlling thefocus of said at least one ion beam so as to modify the roughness ofsaid surface.
 10. A method according to claim 9, wherein a first focusis used for forming said data structure, while a second focus is usedfor forming a servo pattern.
 11. A method according to claim 1, whereina whole disk is patterned in said ion beam projection and processingsteps.
 12. A method according to claim 1, further comprising the step offorming said mask by an e-beam lithography and a subsequentsemiconductor etching.
 13. A domain expansion storage medium in which amagnetic wall is displaced in a readout layer to thereby enlarge amagnetic domain of a storage layer so as to reproduce the informationindicated by said magnetic domain, said storage medium comprising anintermediate surface processed by ion beam projection lithography with apredetermined pattern to define at least one of a data pattern of saidstorage layer, a track pattern and a servo pattern.
 14. A storage mediumaccording to claim 13, wherein said processed intermediate surfacecorresponds to the surface of a substrate of said storage medium.
 15. Astorage medium according to claim 13, wherein said processedintermediate surface corresponds to the surface of a seed or dielectriclayer deposited on a substrate of said storage medium.